The beneficiation technology and difficulties of copper smelting slag are all valuable information!

Types of copper smelting slag

In the process of pyrometallurgical copper smelting, crude copper or anode copper is generally produced through three procedures: smelting, blowing and refining. Anode copper is refined through electrolysis to become electrolytic copper. The blowing slag is returned to the smelting process, and the refining slag is returned to the blowing process. Some factories, based on process requirements, configure pyrometallurgical dilution procedures for smelting slag and blowing slag, thus resulting in the production of depleted slag. The traditional equipment used in smelting furnaces includes blast furnaces, reverberatory furnaces, electric furnaces, etc. The newly-built modern large-scale copper smelting plants mostly adopt relatively advanced processes, which can be summarized into two categories. One is the suspension smelting process, such as the rotary float smelting of Xiangguang Copper Industry, the flash smelting of Autokunpu, the oxygen flash smelting of INCO and the CONTOP continuous top-blowing smelting of German KHD Company, etc. Another category is the pool smelting process, such as the Noranda smelting process, the Mitsubishi process, the Vanukov process, the Isa process and the silver process, etc. The blowing furnaces mainly adopt horizontal converters, with a few using siphon converters, Mitsubishi process blowing furnaces and continuous blowing furnaces. The flash blowing jointly developed by Outokumpu and Kennecott and the rotary floating blowing developed by Xiangguang Copper Industry have been successfully applied in industrial production and are gradually becoming the mainstream trend. Rotary refining furnaces are widely used in refining, with only a very small number employing reverberatory furnaces.

Copper smelting slag, also known as copper slag, is classified into pyrometallurgical copper smelting slag and hydrometallurgical copper smelting slag based on different processing methods. Pyrometallurgical copper smelting slag is also called copper smelting furnace slag or copper metallurgical furnace slag, while hydrometallurgical copper smelting slag is also known as copper leaching slag or copper impregnation slag. According to the pyrometallurgical process, it can be further classified into smelting slag, blowing slag, refining slag and dilution slag. According to different equipment, it can be divided into blast furnace slag, flash furnace slag, electric furnace slag, converter slag and reflector slag, etc. According to the different cooling methods of the slag, it can be classified into water-quenched slag, naturally cooled slag, heat-held cooled slag, slag ladle slowly cooled slag and slag cast by casting machine, etc.

The impoverishment of copper smelting slag is carried out through pyrometallurgical impoverishment and beneficiation impoverishment. Pyrometallurgical impoverishment mostly employs electric furnace method and reverberatory furnace method. In addition, there are vacuum impoverishment method, slag bucket method, molten salt extraction method, etc. The slag produced by pyrometallurgical dilution is called waste slag. Most of the smelting slag is depleted by pyrometallurgy, while only a few are depleted by beneficiation. However, the slag from blowing is mostly processed through beneficiation and dilution, and many factories no longer return it to the smelting furnace. In recent years, due to the very obvious industrial advantages of the copper smelting slag beneficiation dilution process, such as low cost, good effect, energy conservation and environmental protection, it has gradually shown a development trend of replacing the pyrometallurgical dilution process. At present, the slag used for beneficiation and dilution includes smelting slag, blowing slag and dilution slag.

2. Properties and phase composition of copper smelting slag

Copper smelting slag is a product of pyrometallurgy. Its composition mainly comes from the slag-forming components in the ash of ores, fluxes and reducing agents, and the composition is very complex. In general, slag is a melt of various oxides, which can form compounds under different compositions and temperature conditions, with small amounts of sulfides, nitrides, sulfates, etc. Some of these salts come from raw materials, while others are added as fluxes.

Under normal circumstances, the copper content in the blowing slag is relatively high, followed by the smelting slag. The grade of the depleted slag after dilution is the lowest, generally below 1%. Copper smelting slag is mostly black or brown in color, with a metallic luster on the surface. Its internal structure is basically glassy, dense, hard and brittle, and its chemical composition is relatively complex. The content of iron, silicon dioxide, calcium oxide and aluminum oxide in the slag is relatively high, accounting for more than 60%. Due to the different sources of copper ores, in addition to copper, they also contain valuable elements such as cobalt and nickel. Usually, they also contain valuable metals such as lead, zinc, gold and silver, but in relatively low amounts. In terms of content range, the iron content is approximately 30% to 45%, and the copper content varies in some cases, ranging from 1% to 2%, which falls within the range of low-grade copper ores; in others, it is between 1% and 2%, which is classified as medium-grade copper ores; and in others, it is above 2%, which is categorized as rich-grade copper ores. The vast majority of the mineral composition is iron olivine, followed by magnetite, and there is also a small amount of vitreous body composed of gangue. Among them, the copper minerals, due to different smelting processes, are distributed in the slag in various forms such as copper oxide, copper sulfide, metallic copper, and combined copper, as well as in different contents. In addition, some smelters, due to the special nature of the copper ore raw materials they process, produce slag that contains valuable metals such as gold, silver, and cobalt that can be recycled.

Copper smelting slag, in a broad sense, is a kind of "artificial ore", and its material composition structure varies with the conditions of the smelting process. Copper smelting slag is a substance with a relatively complex composition, generally containing 5 to 6 or more kinds of oxides, various sulfides, sulfates and other trace components. The appearance is generally black or dark green, dense and hard, with a specific gravity of about 4. The most abundant components in the slag are iron and silicon. The main minerals are iron olivine and magnetite, along with a small amount of pyrrhotite. Most of the silicon forms iron silicates in the slag, and a small amount of silicon appears as wollastonite and opaque glassy substances. Secondly, there are copper sulfides, metallic copper and a small amount of copper oxide, etc. It also contains a very small amount of valuable components such as gold, silver, nickel and cobalt, mainly distributed in magnetic iron compounds and iron silicates, existing in the form of ferrous silicates or silicates.

Copper in slag mostly exists in the form of sulfides, mainly including chalcocite, chalcocite, chalcopyrite, chalcocite and metallic copper, etc. Copper minerals are often interwoven with the iron olivine matrix and magnetite in the slag, or are spherical and wrapped by magnetite. Some are porphyritic structures jointly formed by copper and iron minerals in the iron olivine matrix, or several copper minerals are intercalated and coexist. The particle size of copper and iron minerals in copper slag varies greatly with the cooling conditions of the slag and the composition of the slag. Iron in slag has a strong affinity for oxygen, forming iron oxide. A part of it combines with silicon dioxide in the slag to form low-melting-point iron olivine, which is the matrix of the slag, accounting for more than 28% of the total iron content of the slag. The remaining part is Fe304, accounting for more than 30% of the total iron content of the slag, and there are also small amounts of Fe2O3 and FeS. In addition to forming iron olivine with FeO, silicon dioxide in the slag also forms a small amount of wollastonite and glass phase with CaO and others.

The typical components of copper smelting slag are Fe: 29-40%, SiO2:30-40%, Al2O3: ≤10%, CaO: ≤11%, Cu: 0.42-4.6%. The composition varies among different smelting methods, and the composition and content of copper and iron phases in different smelting slags also differ. The copper grade of the smelting slag is between 0.76 and 4.58%, and the iron grade is between 40.92 and 45.99%. The copper phase in copper smelting slag is mainly composed of copper sulfide, which accounts for over 60% of the total copper distribution, with the highest reaching 87.13%. Next come copper oxide and metallic copper, with copper oxide ranging from 5% to 60%, and the highest reaching 56.55%. The proportion of metallic copper is between 9% and 25%, with a high of 20.31%. The proportion of other copper in some slag exceeds 10%. In the iron phase, iron olivine and magnetite are the main components, and the sum of the two accounts for more than 80% of the total iron distribution. Among them, iron olivine is between 8% and 65%, with the highest reaching 62.14%. Magnetite is between 33% and 45%, with the highest reaching 44.09%. The distribution rate of individual hematite is relatively high, reaching 57.43%.

The slag from the blowing furnace is mainly converter slag, which is produced by blowing ice copper in a converter. The appearance of converter slag is black or black with a hint of green. It is brittle, hard and has a dense structure, with a density of approximately 4 to 4.5t/m ³. The most abundant elements in the slag are iron and silicon, both of which exist in a combined state. The main components are iron olivine and magnetite. Water-quenched copper slag is a black, dense, hard and wear-resistant glassy phase. It appears in granular and strip-like forms, with a small amount of needle-like and flaky substances mixed in. The surface has a metallic luster, and the particle shape is irregular with distinct edges and corners. Its density is 3.3 to 4.5t/m ³, and the bulk density is 1.6 to 2.0 t/m ³. The porosity is about 50%, the fineness modulus is 3.37 to 4.52, and it belongs to coarse sand type slag. In addition, if the slag contains trace amounts of toxic elements, toxic organic substances, and radioactive materials, but does not have any of the following properties: leaching toxicity, corrosiveness, radioactivity, or acute toxicity, it is classified as general solid waste and can be subject to development research. Due to different process conditions, there are certain differences in the composition of the phases.

The copper grade of the blowing converter slag is between 1.74% and 7.26%, and the iron grade is between 49.45% and 53.59%. The copper phase in copper converter slag is mainly composed of copper sulfide and metallic copper, and the sum of the two accounts for more than 78% of the total copper distribution. Among them, copper sulfide is between 44% and 96%, with the highest reaching 90.4%. Copper, on the other hand, ranges from 15% to 36%, with the highest reaching 35.14%. Copper oxide ranges from 10% to 20%, with the highest reaching 19.82%. In the iron phase, iron olivine and magnetite are the main components, and their combined proportion accounts for more than 50% of the total iron distribution. Iron olivine is between 80% and 70%, with the highest reaching 69.88%. Magnetite is between 26% and 80%, with the highest reaching 78.18%.

3. Beneficiation methods for copper smelting slag

Copper smelting slag, in most cases, contains recyclable ferrous metals, non-ferrous metals, and rare and precious metals, etc., and is often a mixture with significant variations in composition. If one wants to recover various valuable minerals from copper smelting slag, multiple beneficiation methods can be adopted. The specific slag beneficiation method should be determined based on the nature of the slag and the types of recyclable metals. Overall, the beneficiation methods for copper smelting slag include various beneficiation methods such as flotation beneficiation, magnetic beneficiation, gravity beneficiation, chemical beneficiation and combined beneficiation. Each of the above mineral processing methods is further classified into more mineral processing methods based on different combinations of specific processes and equipment.

The minerals in copper smelting slag containing non-ferrous metals are generally highly floatable, and the flotation beneficiation method is often used to recover non-ferrous metals. For instance, smelting slag containing copper, lead, zinc and other metals all adopt the flotation method. Some slags contain copper oxide, metallic copper, gold and silver minerals. Besides flotation, based on the differences in specific gravity and chemical properties of copper, gold, silver and other minerals, gravity separation or chemical beneficiation methods can be selected for treatment. Some copper smelting slag contains iron and cobalt minerals. As both have strong magnetic properties, magnetic separation methods can be chosen for recovery. Slag from smelting containing rare and precious metal minerals is generally recovered by gravity separation and chemical separation based on the specific gravity differences and chemical properties of the metal minerals. For instance, gold in gold-bearing slag can be recovered by gravity separation, or by chemical beneficiation methods such as mercury amalgamation or chemical leaching. It can also be recovered by flotation taking advantage of the floatability of gold-bearing minerals. Generally, combined beneficiation methods are mostly adopted.

At present, worldwide, based on the properties of copper smelting slag, the mature beneficiation processes applied in production practice basically take copper and iron as the main objects of recovery. According to the characteristics of mineralogical slag properties, flotation and magnetic separation are usually adopted as beneficiation methods. For slag containing rare and dispersed metal minerals and refractory minerals, chemical beneficiation or combined beneficiation processes should be adopted for comprehensive recovery and treatment.

4. The significant value and future development prospects of copper smelting slag selection technology

Copper smelting slag beneficiation technology is an energy-saving and environmentally friendly technology, which has very important economic and environmental protection value.

In the past, when the copper smelting converter slag was returned to the smelting furnace for re-smelting, due to the increased viscosity of the smelting slag, the separation conditions of the ice copper and the slag deteriorated, resulting in a decline in the comprehensive smelting indicators. Later, with the continuous optimization and upgrading of the world's copper metallurgical technology, the copper smelting slag beneficiation technology was applied and developed in the copper smelting process. After gradually replacing the copper slag pyrometallurgical dilution process, the amount of slag returned was reduced, greatly reducing the occupied area of the furnace bed and eliminating the adverse effects of iron (III) oxide on smelting.

The electric furnace dilution process is adopted, with the copper content in the waste slag reaching as high as 0.5% to 0.6%, and the power consumption reaching 145 kw.h/t, causing serious environmental pollution. Before 1996, the Finnish company Outokunpu used the electric furnace dilution method to treat flash melting slag and blowing slag. The copper content in the discarded slag was 0.5% to 0.7%, and the copper recovery rate was 77%. However, after switching to the beneficiation method, the copper content in the slag was 0.3 to 0.35%, and the copper recovery rate increased to 91.1%. During the trial production of the Noranda furnace in Daye, the Noranda smelting slag was depleted in a reverberatory furnace, and the average copper content of the discarded slag was 0.73%. However, after switching to beneficiation for depletion, the copper content of the slag dropped below 0.35%. The copper recovery rate is as high as over 94%. For Outokumpu Company, the power consumption during the impoverishment process using an electric furnace is 90kwh per ton of slag, while it is 44.2kwh per ton of slag in the beneficiation method. In terms of energy conservation and improving copper recovery rate, slag beneficiation technology has outstanding advantages such as energy conservation and high recovery rate compared with conventional pyrometallurgical methods.

In terms of environmental protection and resource utilization, compared with pyrometallurgical dilution, the beneficiation dilution technology is relatively cheaper in both infrastructure investment and equipment maintenance. Pyrometallurgical dilution generates low-concentration SO2 flue gas, which cannot be economically treated and is directly discharged into the atmosphere, seriously polluting the environment. The beneficiation method is generally carried out under normal temperature and pressure and in weakly alkaline media. As long as the treatment and reuse of flotation wastewater are properly addressed, "zero pollution" to the environment can be achieved. The pyrometallurgical dilution process is limited to the recovery of copper metal, while the slag beneficiation technology can not only be used as a dilution process for copper slag to recover copper minerals within it, but also for the resourceful recovery of other valuable resources in the copper slag. The resource utilization and harmless treatment of copper smelting slag, the reduction of land occupation and the promotion of sustainable development of enterprises are the basic national policies of China to ensure the healthy development of enterprises, and also the current trend of the world's development. Since the implementation of the access system for the copper smelting industry in China in 2006, the important position of slag beneficiation and dilution technology in the copper smelting industry has been basically established, gradually leading the slag dilution process of China's copper smelting enterprises onto the path of beneficiation and dilution technology. Therefore, the replacement of the pyrometallurgical dilution process by copper smelting slag beneficiation technology is an unstoppable trend of The Times.

China is a major consumer and importer of copper. Since the reform and opening up, copper production has been growing rapidly. In 2000, China's copper output reached 1.32 million tons, surpassing Chile to become the world's largest producer. In 2010, China's copper output increased to 4.79 million tons, and in 2020, it soared to 1,003 tons. The output of copper smelting slag in 2020 reached approximately 30 million tons. Significant progress has been made in domestic copper smelting slag beneficiation technology, which can reduce the copper content of copper smelting slag to below 0.25%, reaching the world's leading level. Based on the annual production of over 30 million tons of domestic copper smelting slag, for every 0.1 percentage point reduction in the grade of copper smelting slag through copper smelting slag beneficiation technology, calculated at the current copper price of 58,000 yuan per ton, an additional economic benefit of approximately 1.74 billion yuan will be created. Copper slag contains nearly 40% iron. If one more percentage point of iron is recovered, the wealth it creates will also be considerable.

Copper smelting slag contains a large amount of valuable elements such as copper, iron, cobalt, nickel, lead, zinc and silicon. At present, the country's copper resources are seriously insufficient, and it has become highly dependent on foreign minerals. China's steel industry is developing rapidly, and the demand for iron ore has soared. However, domestic supply is far from meeting the needs of the steel industry. The price of iron ore in the international market has soared for several consecutive years, which has affected the healthy development of China's steel industry. In recent years, Chinese scientific and technological personnel have carried out a large amount of research work and achieved gratifying results in the comprehensive recovery of qualified grade iron concentrate, ferroalloys and accessory metals from copper slag modification. Therefore, it is not difficult to predict that the beneficiation technology of copper smelting slag will have a very good application prospect.

China's slag beneficiation technology has entered a stage of comprehensive development. As a result, the monograph "Slag Beneficiation for Copper Smelting" emerged, promptly meeting the theoretical and learning needs of China's copper smelting development and receiving high praise from the majority of technicians and managers in the copper smelting industry.

Copper smelting slag beneficiation technology is currently mainly applied to converter slag, electric furnace slag or a mixture of both, as well as flash melting furnace slag. Some enterprises, taking advantage of their regional strengths, sell the tailings after slag beneficiation to cement plants in the form of cement fillers, achieving tail-free production. But the majority